a) Field of the Invention
This invention relates to polymerizable compounds having the norbornanelactone structure and also to polymers having the same structure and useful as materials for electronic industry such as photoresists.
b) Description of the Related Art
Keeping in step with a demand for still further densification and integration of semiconductor devices in recent years, there is an increasing move toward use of an exposure light source of a shorter wavelength in a photolithographic step. Described specifically, in place of conventionally-employed g-beam (436 nm) and i-beam (365 nm), KrF excimer laser (248 nm) is now coming to constitute a mainstream, and a future move to ArF excimer laser of a still shorter wavelength (193 nm) is under investigation.
As a base polymer of a resist material for use in photolithography, use of a novolak resin has been primarily recommended when g-beam or i-beam is employed as a light source while use of a hydroxystyrene-base resin has been recommended when KrF excimer laser is employed as a light source, because these phenolic resins are excellent in various properties required as resist materials such as post-exposure alkali solubility, dry etching resistance and substrate adherence. Phenolic resins are however not considered to be usable as resist materials when ArF excimer laser is employed as a light source, because around 193 nm, each of them exhibits an extremely strong absorption of light, said absorption being ascribed to the aromatic ring.
A variety of resins containing no aromatic rings in their structures were therefore investigated as resist materials which permit use of ArF excimer laser as a light source. For example, processes making use of a specific acrylic-acid-base or methacrylic-acid-base resin have been proposed.
When a resin containing no aromatic rings therein is used as a resist material, on the other hand, another problem arises in that high dry etching resistance attributed to the existence of aromatic rings as in conventional phenolic resins is not available. As a method for resolving this problem, it has been proposed to introduce alicyclic groups, such as adamantane skeletons, norbornane skeletons or a terpene, in an acrylic-acid-base or methacrylic-acid-base resin. Illustrative of resist materials available by these processes are those disclosed in JP 5-265212 A1, JP 7-199467 A1, JP 8-82925 A1, etc.
The introduction of alicyclic groups into a resin leads to an improvement in dry etching resistance, but a problem arises in that, because the hydrophopicity of the resin becomes stronger, substrate adherence becomes insufficient and pattern peeling takes place during development. Accordingly, it has also been proposed to retain substrate adherence by introducing a lactone ring, which has polarity, in a monomer to be copolymerized. Illustrative of resist materials available by these processes are those disclosed in J. Photopolymer Science and Technology, 9(3), 475-487, 1996, ibid., 9(3), 509-522, 1996, ibid., 10(4), 545-550, 1997, JP 9-90637 A1, etc.
In the field of photolithography where there is a move toward shorter wavelengths, development of corresponding resist materials is essential. It is, therefore, the current circumstance that seeking for base polymers capable of simultaneously satisfying properties such as transparency at the above wavelengths, dry etching resistance and substrate adherence, research is under way by trial and error.
There is, accordingly, a long standing desire for the development of base polymers which can satisfy all of these properties.
An object of the present invention is to provide a polymer suitably usable as a base polymer for a resist which permits use of ArF excimer laser as a light source, that is, a novel polymer capable of satisfying performance such as transparency at 193 nm, dry etching resistance and substrate adherence, and also a polymerizable compound required for the production of the polymer. Another object of the present invention is to provide a positive resist composition of the chemical amplification type, which makes use of the above-described polymer.
The present inventors have proceeded with extensive research to achieve the above-described objects. As a result, it has been found that a polymer, which contains in side chains thereof specific norbornanelactone structures represented by the below-described formula (2), has high transparency at a short wavelength such as ArF excimer laser and excellent dry etching resistance and substrate adherence and is useful as a polymer for a resist, especially as a polymer for a positive resist. This finding has led to completion of the present invention.
Specifically, the present invention provides a polymerizable compound represented by the following formula (1): 
wherein one of R1 and R2 represents an alkyl group, which has 1 to 4 carbon atoms and is other than a tert-butyl group, and the other represents a hydrogen atom or an alkyl group which has 1 to 4 carbon atoms and is other than a tert-butyl group; R3 to R9 each independently represent a hydrogen atom, a silyl group, a cyano group, a hydroxyl group, a halogen atom or a monovalent nonpolymerizable organic substituent group; and A represents a polymerizable group having a carbon-carbon double bond.
Further, the present invention also provides a polymer having, in at least one side chain thereof, a group represented by the following formula (2): 
wherein one of R1 and R2 represents an alkyl group, which has 1 to 4 carbon atoms and is other than a tert-butyl group, and the other represents a hydrogen atom or an alkyl group which has 1 to 4 carbon atoms and is other than a tert-butyl group; and R3 to R9 each independently represent a hydrogen atom, a silyl group, a cyano group, a hydroxyl group, a halogen atom or a monovalent nonpolymerizable organic substituent group.
In addition, the present invention also provides a polymer obtained by polymerizing or copolymerizing the polymerizable compound represented by the formula (1).
Moreover, the present invention also provides a positive resist composition comprising one of the above-described polymers and an acid yielding agent which gives off an acid when exposed to light.
As the polymer according to the present invention does not contain any aromatic ring and has a specific norbornanelactone structure, it is excellent in substrate adherence and dry etching resistance and is suitably usable as a resist material for an exposure light source of a shorter wavelength such as ArF excimer laser.